1. Field
The present invention relates to a spectrometer comprising a spatial light modulator (SLM) such as a digital micro-mirror device (DMD).
2. Description of Related Art
Spectrometers are employed in the analysis of wavelength dependent intensity variations of optical radiation, from the ultraviolet to the infrared spectral regions. Typically, a dispersion element such as a prism or a diffraction grating is employed in these spectrometers to disperse incident optical radiation by wavelength in a preferred dispersion plane. An input is provided comprising an entrance field stop, typically an entrance slit, acting as a bandpass limiter for the optical radiation which is to be incident on the dispersion element. This field stop essentially determines the optical resolution and throughput of the spectrometer.
As is well known, the dispersion element may be moved, typically rotated about an axis perpendicular to the dispersion plane, in order to sweep individual wavelengths of a wavelength region of interest of the dispersed optical radiation sequentially over an output which may be a detector, an exit slit or other optical radiation collector. This places significant precision requirements on the mechanical system employed to effect the movement of the, often heavy, dispersion element and such systems are known to be susceptible to external mechanical disturbances and wear.
One known solution to this problem is to provide spectrometer having a static dispersion element and incorporating a detector array of separately addressable elements in place of the single detector typically employed in conjunction with the movable dispersion element. The stationary dispersion element operates to disperse a wavelength region of interest which is here distributed by wavelength across the elements of the detector array in the dispersion plane. However, signal detection requires sophisticated and relatively expensive electronics and the detector arrays are themselves relatively expensive, particularly for detector arrays suitable for the detection of wavelengths in the infrared region.
Furthermore, spectrometers which comprise an SLM are known from for example U.S. Pat. No. 5,504,575 which is assigned to Texas Instruments Incorporated, address both the problems of mechanical movement of the dispersion element and the use of a detector array. According to the known SLM spectrometer there is provided an input by means of which is illuminated a stationary prism, grating or other type of wavelength dispersion element, typically having a preferred dispersion plane. An SLM, such as a DMD, a magneto-optic modulator or a liquid crystal device, is provided to receive, distributed by wavelength across its active surface, an entire wavelength region of interest having been dispersed in the preferred dispersion plane by the dispersion element. By activating (or deactivating) small portions (i.e. cells) of its active surface the SLM is operable to selectively direct a wavelength portion of the received wavelength region of interest to the output. Through appropriate activation and deactivation of the individual cells or groups of cells (typically groups of cells in a direction perpendicular to the dispersion plane i.e. columns) different narrow wavelength bands of the received wavelength region of interest can be provided to the output. In this manner the entire wavelength region of interest may be swept sequentially across the output and a single detector element may be employed.